1. Field of the Invention
The present invention relates to leuco dyes having a sufficient absorption intensity in a near infrared region, which are capable of yielding colored images when brought into contact with an electron acceptor such as inorganic acids, organic acids, phenolic compounds and derivatives thereof, or oxidizing agents and a recording material using any of the above-mentioned leuco dyes as an electron donor coloring agent.
2. Discussion of Background
Recording materials using leuco dyes are conventionally known and used in practice, for example, as pressure-sensitive recording sheets and thermosensitive recording sheets. The consumption of the above-mentioned sheets shows a yearly increase.
The pressure-sensitive recording materials containing leuco dyes employ the reaction between a colorless or light-colored leuco dye and a color developer which is capable of inducing color formation in the leuco dye when brought into contact with the leuco dye. More specifically, the pressure-sensitive recording material comprises a coloring sheet and a color developer sheet. The coloring sheet is prepared by coating a microcapsuled organic solution of a leuco dye on a substrate, and the color developer sheet is prepared by coating mixture of the color developer and a binder agent on a substrate, separately. The pressure-sensitive recording material is constructed in such a manner that the above mentioned color developer sheet is overlaid on the coloring sheet, with the respective coated surfaces in contact with each other. When pressure is applied to the back side of the color developer sheet, for instance, by writing, the microcapsules of the leuco dye are ruptured to initiate the coloring reaction with the color developer.
Thermosensitive recording materials containing leuco dyes comprise a substrate and a thermosensitive coloring layer formed thereon comprising a leuco dye and a color developer. The thermosensitive recording material can yield colored images in response to thermal image signals applied from a heating resistor.
When the above-mentioned pressure-sensitive recording sheets and thermosensitive recording sheets are employed, recording can be more easily performed by using relatively simple apparatus without any complicated processes such as development and image fixing, in comparison with other recording materials such as electrophotographic recording materials and electrostatic recording materials. Therefore the pressure-sensitive recording sheets and thermosensitive recording sheets are utilized in various fields. Examples of-the leuco dyes contained in the conventional pressure-sensitive recording sheets and thermosensitive recording sheets include blue coloring dyes such as crystal violet lactone and leuco crystal violet, and black coloring dyes such as fluoran compounds substituted by an anilino group at the 7-position.
Recently optical character readers and label-bar code readers are developed and put into practice. These optical character readers and label bar-code readers generally employ, as a light source, a light emitting diode or a semiconductor laser having a wavelength of 700 nm or more. However, the above-mentioned conventional leuco dyes such as blue-coloring dyes and black-coloring dyes hardly show absorption intensity in a near infrared region of 700 nm or more, so that they cannot be used together with the above-mentioned optical character readers and bar-code readers Thus a demand for a novel leuco dye with a sufficient absorption intensity in a near infrared region of 700 nm or more is increasing.
Dyes and pigments showing sufficient absorption intensity in a near infrared region, such as phthalocyanine pigments, quinacridone pigments and chelate compounds of various metals, are conventionally known. These dyes and pigments, however, show high absorption intensity in the visible light range as well, so that they cannot be used for the pressure-sensitive recording materials and thermosensitive recording materials in an ordinary manner. To provide the above-mentioned dyes and pigments for the pressure-sensitive and thermosensitive recording materials, the dyes and pigments must be turned into the form of a leuco compound which is ordinarily colorless or light-colored, but is capable of yielding colored images when brought into contact with an electron acceptor.
There are conventionally proposed several leuco dyes having a sufficient absorption intensity in a near infrared region, for example, divinyl-containing phthalide compounds as disclosed in Japanese Laid-Open Patent Applications 51-121035, 51-121037, 51-121038, 51-167979, 58-157779 and 61-165380; spirofluorenephthalide compounds as disclosed in Japanese Laid-Open Patent Applications 59-199757, 60-226871, 61-22076 and 62-104872; and fluoran compounds as disclosed in Japanese Laid-Open Patent Applications 57-169484 and 62-196177.
However, the above-mentioned conventional leuco dyes have their own shortcomings. For example, the divinyl-containing phthalide compounds are intensely colored in yellow in their original state, and the manufacturing cost thereof is high due to the difficulty in synthesizing them. The spirofluorenephthalide compounds and fluoran compounds have also the shortcomings that their absorption intensity in a near infrared region is weaker than in the visible light range.